Photovoltaic devices are used in several industrial applications to produce electricity under illumination of light. A conventional photovoltaic device has a semiconductor p-n junction and a pair of electrodes. Upon exposure to light, the semiconductor p-n junction converts the optical energy into electrical energy output via the electrodes.
Photovoltaic devices of the above kind, however, may only produce a photovoltaic voltage below a couple of volts, due to the fundamental limitation of the semiconductor photovoltaic effect. To obtain a higher photovoltage, ferroelectric materials with aligned spontaneous electric polarization have been the subject of research since the 1970's. Examples of ferroelectric photovoltaic materials are the bulk ferroelectric ceramic and single crystal materials. This type of devices is capable of producing a voltage of several hundred volts.
Because bulk ferroelectric ceramic and single crystal materials are difficult to be integrated into micro systems, such as micro electromechanical systems (MEMS), researchers have expanded their efforts to focus on ferroelectric thin films since the 1990's. FIG. 1 shows a conventional ferroelectric thin film photovoltaic device. The device is comprised of a substrate 10, a bottom electrode 13 formed on substrate 10 via intermediate layers 11 and 12, a ferroelectric layer 14 formed on bottom electrode 13 and one or more top electrodes 15 formed on ferroelectric layer 14. An electric field is applied to ferroelectric layer 14 through top and bottom electrodes 15 and 13, to pole the ferroelectric layer 14. The ferroelectric polarization 17 is therefore aligned perpendicular to the ferroelectric layer 14. When exposed to light 16, the photovoltage device generates a photovoltage between top and bottom electrodes.
The photovoltage generated by this type of device is very low, typically only below one volt. To increase photovoltage, the thickness of the ferroelectric layer 14 may be increased. The consequence of increasing the thickness of the ferroelectric layer is that, the fabrication of the device becomes more complicated and at higher cost. The overall volume of the photovoltaic device also increases, which are not desirable characteristics in micro systems or micro devices, such as MEMS.
As such, ferroelectric thin film photovoltaic devices, much like the semiconductor photovoltaic device, are limited to producing a photovoltage of below one volt. In many applications, particularly involving mechanical movement, such as MEMS, this low-level of voltage may be insufficient. Researchers, therefore, seek to provide a ferroelectric thin film photovoltaic device capable of providing higher photovoltages, without increasing the thickness of the ferroelectric layer, which are suitable for applications such as MEMS or the like.